FIG. 4 is a block diagram showing an example of known multicarrier feed-forward amplifiers. A multicarrier feed-forward amplifier 100 (hereinafter referred simply as “multicarrier amplifier 100”) shown in FIG. 4 amplifies a main signal having a plurality of carrier signals at frequencies (RF (Radio Frequency)) differing from one another, in common with the carriers, while compensating a distortion component (harmonic component) generated in the main signal due to the amplification.
As shown in FIG. 4, the multicarrier amplifier 100 comprises, for example, a main signal canceling unit 100A having a first branch circuit 101, a first variable attenuator 102, a first variable phase shifter 103, a pilot oscillator 104, a first combining circuit 105, a main signal amplifier (HPA: High Power Amplifier) 106, a second branch circuit 107, an attenuator 108, a first delay line 109 and a second combining circuit 110, a distortion canceling unit 100b having a second variable attenuator 111, a second variable phase shifter 112, an auxiliary amplifier 113, a second delay line 114 and a third combining circuit 115, along with third and fourth branch circuits 116 and 117, a local oscillator for frequency conversion 118, a frequency converter 119, a band-pass filter (BPF: Band Pass Filter) 120, a received signal strength indicator (RSSI; Received Signal Strength Indicator) for a pilot signal 121, a received signal strength indicator (RSSI) for all carrier signals 122, and a control unit 123.
In the main signal canceling unit (distortion extracting unit) 110A, the first branch circuit 101 takes out a part of an input signal having a plurality of carriers (for example, frequencies fRF1−fRFn; n being an integer not less than two) and outputs it acting as an auxiliary signal for canceling a main signal to the delay line 109, besides outputting the remaining input signal acting as the main signal to the variable attenuator 102. The variable attenuator 102 and the variable phase shifter 103 invert and adjust the phase of the main signal according to control information from the control unit 123 so that the main signal from the above first branch circuit 101 is combined by the second combining circuit 110 at the same level as the auxiliary signal and in the oppposite phase to that of the auxiliary signal.
The pilot oscillator 104 generates a pilot signal having a predetermined frequency fp. The first synthesizing circuit 105 combines the main signal, the phase and level of which have been adjusted, and the pilot signal from the pilot oscillator 104, thereby adding the pilot signal to the main signal. The main signal amplifier 106 amplifies the main signal, to which the pilot signal has been added as above, in common with the carriers.
The second branch circuit 107 branches an output of the above main signal amplifier 106 into two signals, one which is outputted as an output of the multicarrier amplifier, the other of which is outputted as a signal to be combined with the above auxiliary signal. The attenuator 108 attenuates the level of the signal from the branch circuit 107 to a level before being amplified by the main signal amplifier 106.
The second combining circuit 110 combines a signal (the phase of which has been inverted by the variable phase shifter 102) from the attenuator 108 and the above auxiliary signal taken out by the first branch circuit 101, thereby canceling a main signal component in a signal from the second branch circuit 107. As a result, only a distortion component generated in an amplified output (main signal) due to the linear characteristic of the main signal amplifier 106 is detected. The delay line 109 delays the above auxiliary signal by a predetermined time so that the timing of the auxiliary signal and the timing of a signal from the second branch circuit 107 are matched in the second combining circuit 110 when they are combined.
In the distortion canceling unit 100b, the variable attenuator 11 and the variable phase shifter 112 adjust the level and phase of the distortion component generated in an output of the main signal amplifier 106 using the distortion component detected by the main signal canceling unit 100a (second combining circuit 110) as above according to control information from the control unit 123 in order to cancel the distortion component. The auxiliary amplifier (distortion signal amplifier) 113 amplifies the gain of the distortion component, the level and phase of which have been adjusted, and obtains a gain necessary to cancel the distortion of the main signal in the third combining circuit 115.
The third combining circuit 115 combines an output of the main signal amplifier 106 and an output of the auxiliary amplifier 113, thereby canceling the distortion component generated in the output of the main signal amplifier 106. Each of the third and fourth branch circuit 116 and 117 takes out a part of the main signal in which the distortion component has been canceled. A part of the main signal taken out by the third branch circuit 116 is outputted to the frequency converter 119. A part of the main signal taken out by the fourth branch circuit 117 is outputted to the received signal strength indicator 122.
The local oscillator 118 fixedly generates a frequency signal in IF (Intermediate Frequency) band. The frequency converter 119 converts (down-converts) the main signal in RF band from the third branch circuit 116 to a frequency signal in IF band using the frequency signal from the local oscillator 118. The BPF 120 allows only a pilot frequency component of the main signal down-converted as above to pass therethrough to extract a pilot signal from the main signal. The above down-converting process is performed mainly to relief the band-pass (cut-off) characteristic that the BPF 120 is required.
The received signal strength indicator 121 for a pilot signal detects a received signal strength of the pilot signal obtained by the above BPF 120, thereby to detect a power level of the pilot signal. Namely, part comprising the local oscillator 118, the frequency converter 119, the BPF 120 and the received signal strength indicator 121 forms a pilot power detecting unit 124 exclusive to the pilot signal.
Incidentally, the received signal strength indicator 122 for all carriers detects the received signal strength, not of each carrier but collectively, of the main signal taken out by the above fourth branch circuit 117, thereby detecting a total power level of the main signal.
The control unit 123 controls each of the variable attenuators 102 and 111, and the variable phase shifters 103 and 112 on the basis of a power level detected by the received signal strength indicator 121 for a pilot signal to adaptively control a distortion canceling (compensating) operation so that the distortion component of the main signal to be finally outputted become minimum. On the other hand, the control unit 123 monitors whether the total power level of the carriers detected by the received signal strength indicator 122 for all carriers exceeds a predetermined threshold value or not. When the total power level exceeds a predetermined over-output threshold value, the control unit 123 controls to disconnect (shutdown) the power supply in order to protect the device. Incidentally, the control unit 123 is configured with, for example, a CPU (Central Processing Unit) or the like.
In the known multicarrier amplifier 100 with the above structure, an output of the main signal amplifier 106 and an auxiliary signal taken out by the first branch circuit 101 are combined at the same level and in opposite phases in the second combining circuit 110 of the main signal canceling unit 100a, a main signal component is canceled in an output of the main signal amplifier 106, a distortion component is extracted, the distortion component and the output of the main signal amplifier 106 are combined at the same level and in the opposite phases in the combining circuit 115 of the distortion canceling unit 100b, whereby the distortion component of the main signal is canceled.
The known multicarrier amplifier 100 can measure a total power of the carriers by the received signal strength indicator 122, but it is very difficult for the multicarrier amplifier 100 to measure each carrier power. In order to confirm a carrier power level of one wave, it is necessary to stop other carriers or use an expensive, exclusive spectrum analyzer (device that can evaluate a channel power), for example.
When it becomes necessary to increase the number of carriers in order to cope with an increase in subscribers, the present condition in, for example, a multicarrier amplifier 100 used in a transmission system of a mobile communication base station in CDMA (Code Division Multiple Access) system is that it is necessary to stop all existing carriers and to stop services under operation, and manually adjust a power of an increased carrier (new or added carrier) while confirming it.
Particularly, in CDMA system, when the carrier transmission power of one wave is extremely large, interference of this wave to other carriers becomes large. For this, there is a severe requirement to adjust an increased carrier to a specified power, thus manual adjustment of the power level requires great human labor.
In the light of the above problem, an object of the present invention is to provide a multicarrier amplifying device which can detect each carrier power and automatically adjust the power level of each carrier without stopping services under operation and without using any special device such as a spectrum analyzer or the like.